Spin transfer MRAM device with separated CCP assisted writing

ABSTRACT

A spin-transfer MRAM is described that has two sub-cells each having a conductive spacer between an upper CPP cell and a lower MTJ cell. The two conductive spacers in each bit cell are linked by a transistor which is controlled by a write word line. The two CPP cells in each bit cell have different resistance states and the MTJ cell and CPP cell in each sub-cell have different resistance states. The MTJ free layer rotates in response to switching in the CPP free layer because of a large demagnetization field exerted by the CPP free layer. An improved circuit design is disclosed that enables a faster and more reliable read process since the reference is a second MTJ within the same bit cell. When R MTJ1 &gt;R MTJ2 , the bit cell has a “0” state, and when R MTJ1 &lt;R MTJ2 , the bit cell has a “1” state.

This is a Continuation application of U.S. patent application Ser. No.11/986,375, filed on Nov. 21, 2007, which is herein incorporated byreference in its entirety, and assigned to a common assignee.

FIELD OF THE INVENTION

The invention relates to a spin-transfer MRAM cell in which an upper CPPcell is separated from a lower MTJ cell by a conducting layer in orderto form different read and write paths, and each bit cell has twospin-transfer MRAM cells that are written into opposite resistancestates.

BACKGROUND OF THE INVENTION

Magnetoresistive Random Access Memory (MRAM), based on the integrationof silicon CMOS with magnetic tunnel junction (MTJ) technology, is astrong candidate to provide a dense (8-25F² cell size), fast (1˜30 nsread/write speed), and non-volatile storage solution for future memoryapplications. The MTJ utilizes a thin dielectric insulating layer likeAlOx, AlNxOy, or MgOx that is formed between a first ferromagnetic layerwhich is pinned in a certain direction by an adjacent anti-ferromagnetic(AFM) layer, and a second ferromagnetic (free) layer. The pinned layerhas a magnetic moment that is fixed in the “y” direction, for example,by exchange coupling with the adjacent AFM layer that is also magnetizedin the “y” direction. The free layer has a magnetic moment that iseither parallel or anti-parallel to the magnetic moment in the pinnedlayer. The tunnel barrier layer is thin enough that a current through itcan be established by quantum mechanical tunneling of conductionelectrons. The magnetic moment of the free layer may change in responseto external magnetic fields and it is the relative orientation of themagnetic moments between the free and pinned layers that determines thetunneling current and therefore the resistance of the tunnelingjunction. When a sense current is passed from the top electrode to thebottom electrode in a direction perpendicular to the MTJ layers, a lowerresistance is detected when the magnetization directions of the free andpinned layers are in a parallel state (“1” memory state) and a higherresistance is noted when they are in an anti-parallel state or “0”memory state.

A MRAM device is generally comprised of an array of parallel firstconductive lines on a horizontal plane, an array of parallel secondconductive lines on a second horizontal plane spaced above and formed ina direction perpendicular to the first conductive lines, and a MTJ cellinterposed between a first conductive line (i.e. word line) and a secondconductive line (i.e. bit line) at each crossover location. In a readoperation, the information stored in a MRAM is read by sensing themagnetic state (resistance level) of the MTJ cell through a sensecurrent flowing top to bottom through the cell in a currentperpendicular to plane (CPP) configuration. During a write operation,information is written to the MRAM by changing the magnetic state in thefree layer to an appropriate one by generating external magnetic fieldsas a result of applying bit line and word line currents in two crossingconductive lines, either above or below the MTJ cell. Thus, the crosspoint of word line and bit line currents is used to program a MTJ cell.In FIG. 1 a, a switching field asteroid is depicted for a conventionalMRAM and in FIG. 1 b, a bit line 1 is shown crossing over a MTJ cell 3and a word line 2 that is below the MTJ cell.

The problem of MTJ cells being disturbed along the same word line andbit lines is a major concern. Switching fields generated by word lineand bit line currents for the conventional MRAM are about 30 to 60 Oe inintensity. The MRAM has to generate a relatively large electric currentmagnetic field to rewrite recorded information. Hence, an electriccurrent of a certain large magnitude should flow through the addresswirings in order to produce a sufficient magnetic field for the writeprocess.

As memory devices are increasingly micro-miniaturized, the addresswiring is also reduced in width so that it becomes difficult to apply asufficient electric current to the address wiring. Additionally,coercive force of the device is increased thereby leading to a greaterelectric current magnetic field and a higher power consumption in thedevice. For this reason, a memory structure that employs magnetizationswitching driven by a spin transfer mechanism is receiving moreattention as a configuration capable of switching the magnetizationdirection by application of a small electric current.

Spin transfer (spin torque) magnetization switching is described indetail by J. Slonczewski in U.S. Pat. No. 5,695,864 and by Redon et al.in U.S. Pat, No. 6,532,164. The spin-transfer effect arises from thespin dependent electron transport properties offerromagnetic-spacer-ferromagnetic multilayers. When a spin-polarizedcurrent transverses a magnetic multilayer in a CPP configuration, thespin angular moment of electrons incident on a ferromagnetic layerinteracts with magnetic moments of the ferromagnetic layer near theinterface between the ferromagnetic and non-magnetic spacer. Throughthis interaction, the electrons transfer a portion of their angularmomentum to the ferromagnetic layer. For example, when spin-polarizedelectrons are passed through a magnetic layer having a particularmagnetic moment in a preferred easy axis direction, these spin-polarizedelectrons will cause a continuous rotation of the magnetic moment vectorwhich may result in a reversal of the magnetic moment vector along itseasy axis. Thus, switching the magnetic moment vector between its twopreferred directions along the easy axis may be effected by passingspin-polarized electrons perpendicularly through the magnetic layer. Thedifference between a Spin-RAM and a conventional MRAM is only in thewrite operation mechanism. The read mechanism is the same.

Recent experimental data from W. Rippard et al. in Phys. Rev. Left., 92,p. 027201[3] (2004) confirms the very essence of magnetic momenttransfer as a source of magnetic excitations, and subsequently,switching. These experiments also confirm theoretical predictions by J.Slonczewski in “Current-driven excitation of magnetic multilayers”, J.Magn. Magn. Materials V 159, L1-L7 (1996), and by J. Sun in Phys Rev. B,Vol. 62, p. 570[5] (2000), stating that the spin-transfer generated nettorque term (Γ) acting on the magnetization under conditions ofspin-polarized DC current is expressed by the equation: Γ=s n_(m×(n)_(s)×n_(m)) where s is the spin-angular momentum deposition rate, n_(s)is a unit vector whose direction is that of the initial spin directionof the current, and n_(m) is a unit vector whose direction is that ofthe free layer magnetization. The above equation indicates that thetorque will have a maximum value when n_(s) is orthogonal to n_(m).

Referring to FIG. 2, a prior art spin-transfer MRAM, also referred to asa Spin-RAM, is depicted from a cross-sectional view. The storage element(MTJ) 10 is formed between a bottom electrode 16 and a top electrode(bit line) 25 and is comprised of an underlayer 17, an AFM layer 18,synthetic anti-ferromagnetic (SyAF) reference layer made of layers19-21, a tunnel barrier layer 22, a free storage layer 23, and a cappinglayer 24. The bottom electrode 16 is connected to a CMOS transistorhaving a source 12, drain 13, and p-type semiconductor substrate 11 thatprovides current for switching the MTJ free layer 23. For data writing,as a current flowing across the storage element from bottom to topreaches a critical current, the magnetization of the free layer 23 willbe written to be anti-parallel to the magnetization direction of thereference layer (i.e. a high resistance state). As a current flowingacross the storage element from top to bottom reaches a criticalcurrent, the magnetization of the free layer 23 will be written to beparallel to that of the reference layer (i.e. a low resistance state).During the read process, a small current flows across the MTJ cell andits resistance is compared with a pre-written MTJ cell (called areference cell) to determine whether it is in a high resistance state orlow resistance state. Typically, the read margin is determined by theratio between the magnetoresistive ratio (dR/R), and the coefficient ofresistance variance (σ/μ) which is the ratio between the resistancestandard deviation σ and the resistance mean value μ.

A critical current for spin transfer switching (Ic), which is defined as[(Ic⁺+I Ic³¹ I)/2], for the present 180 nm node sub-micron MTJ having atop-down area of about 0.2×0.4 micron, is generally a few milliamperes.The critical current density (Jc), for example (Ic/A), is on the orderof several 10⁷ A/cm². This high current density, which is required toinduce the spin-transfer effect, could destroy a thin tunnel barrierlayer such as AlOx, MgO, or the like. In order for spin-transfermagnetization switching to be viable in the 90 nm technology node andbeyond, the critical current density (Jc) must be lower than 10⁶ A/cm²to be driven by a CMOS transistor that can typically deliver 100 A per100 nm gate width. For Spin-RAM applications, the (ultra-small) MTJsmust exhibit a high tunnel magnetoresistance ratio (TMR or dR/R) muchhigher than the conventional MRAM-MTJ that use AlOx as a barrier layer.To apply spin-transfer switching to MRAM technology, it is desirable todecrease Ic (and its Jc) by more than an order of magnitude so as toavoid an electrical breakdown of the MTJ device and to be compatiblewith the underlying CMOS transistor that is used to provide switchingcurrent and to select a memory cell.

Normally, the write current density required to switch free layermagnetization is mainly determined by the MTJ free layer magnetizationmoment, damping ratio, and spin-angular momentum deposition rate whichdepends on the type and quality of the materials used in the MTJ stackof layers. As the device is micro-miniaturized to nanometer scaledimensions, the write current density is unchanged, giving a muchsmaller write current which is scalable to the shrinking MTJ dimensions.Hence, power consumption of the device is reduced. Since write currentalso flows across the MTJ tunnel barrier layer, the reliability of MTJcells becomes a large problem due to the fact that the MTJ will bedamaged as the voltage across a MTJ junction reaches a threshold, theso-called breakdown voltage. In order to solve this problem, a newdesign has been proposed in U.S. Pat. No. 7,149,106 that is illustratedin FIG. 3. During the write process, a magnetic field from a digit line31 rotates the magnetization in a polarizer 39 toward one directionwhich the magnetization of the free layer 37 is written to, and anelectric current 41 flows only from one section 40 a of the write lineacross the polarizer 39 and returns to a second section 40 b of thewrite line through a conducting spacer 38. The write current 41generates a spin-transfer interaction between the magnetic polarizer 39and free layer 37 thereby causing a switch in free layer magnetization.The MTJ cell 30 is also comprised of an AFM/bottom electrode 32, AP2layer 33, Ru coupling layer 34, AP1 layer 35, and a tunnel barrier layer36.

Since the write current does not directly flow into the free layer inthis design, only a very small surface portion of the free layermagnetization may experience a spin-transfer effect. Moreover, theconducting spacer must be made very thin in order to deliver polarizedspin current to free layer magnetization more efficiently. Even so, thespin-transfer effect could still be very small. Thus, a large writecurrent would be required. Additionally, a thin conducting spacer isvery difficult to fabricate and a high current density flowing along thespacer layer would likely cause a reliability problem.

Therefore, an improved design is needed for a Spin-RAM that avoids anoperating voltage which could damage the MTJ tunnel barrier whileproviding a low write current, and high dR/R. Also, as the MRAMdimension shrinks, it is becoming increasingly difficult during a readprocess to differentiate between a “0” and a “1” state when comparingthe resistance in the MTJ cell to a reference cell in the periphery ofthe circuit. Ideally, a better method is needed that does not rely on areference cell outside the bit so that the reliability of the readprocess is improved and the “read” time is minimized.

A routine search of the prior art was conducted and the followingreferences were found. U.S. Pat. No. 7,173,848 discloses a stack of twomemory cells separated by a non-magnetic layer. In related U.S. PatentApplication No. 2006/0202244, there are memory stacks in an X-Y planewhere each stack has two memory cells stacked along a Z-axis direction.

U.S. Pat. No. 5,930,164 shows a stack of two memory cells where thebottom cell is larger and the two cells are separated by a conductivelayer made of Cu.

In U.S. Pat. No. 6,927,948, two differential CPP cells are stackedtogether and are separated by a metal gap layer.

U.S. Patent Application 2006/0146597 discloses a security devicecomprising two paired MRAM cells.

U.S. Pat. No. 7,095,648 describes a matrix of memory cells arranged inrows and columns where one cell in a column (or row) is read whileanother cell in the same column (or row) is written simultaneously.

U.S. Pat. No. 7,009,877 discloses a magnetic memory device having threeterminals in which a spin transfer (ST) driven element is formed betweena first terminal and a second terminal, and a MTJ element is disposedbetween the second terminal and a third terminal. Magnetization reversalof a first free layer within the ST element causes the magnetizationdirection in a second free layer within the MTJ to switch and therebyrecords a data state. However, this reference does not teach how theinteraction between the two free layers can be optimized to improveswitching efficiency. Moreover, the memory device relies on a referencecell outside the bit cell during the read process which can slow thedevice speed.

U.S. Pat. No. 7,230,844 states that a thermal factor represented byK_(U)M_(S)V/k_(B)T where K_(U) is the anisotropy constant, Ms is thesaturation magnetization, V is the volume of the FM2 free layer, k_(B)is the Boltzmann constant, and T is the temperature, should be greaterthan 40 to assure a minimum length for data retention of 10 years in amagnetic material memory element.

SUMMARY OF THE INVENTION

One objective of the present invention is to provide a Spin-RAM designwhich leads to an improved interaction between a free layer in a CPPcell that operates by a spin transfer effect and a free layer in anunderlying MTJ cell to increase read and write efficiency compared withthe prior art.

A second objective of the present invention is to provide a Spin-RAMcircuit employing a plurality of Spin-RAM cells according to the firstobjective to achieve two Spin-RAM cells and two MTJ cells per bit celland allow one MTJ cell to serve as a reference while reading the secondMTJ, thereby avoiding an external reference cell.

According to one embodiment, a more efficient Spin-RAM device isachieved with a bit cell configuration comprised of two CPP/MTJsub-cells in which the two CPP/MTJ cells in each bit pair are written toopposite resistance states represented by (0,1) and (1,0). Each CPP/MTJsub-cell has a stack comprised of a MTJ cell formed on a bottomelectrode, a conductive spacer contacting the capping layer in the MTJ,a CPP cell formed on the conductive spacer, and a bit line contacting acapping layer in the CPP cell. Preferably, a first free layer in the MTJcell has a small anisotropy of less than 5 Oe while a second free layerin the CPP cell has a large uniaxial anisotropy of at least 50 Oe orabout 10× that of the first free layer. Thus, the large demagnetizationfield from the second free layer will easily switch the magnetic momentin the first free layer so that their magnetization directions areanti-parallel and will remain anti-parallel when the second free layermagnetization is switched during a write process. On the other hand, thefirst free layer will exert a relatively small field on the second freelayer so as not to stabilize the CPP free layer. Consequently, the spintransfer switching of the second free layer by a write current can occurmore easily than when the anisotropy of the first free layer and secondfree layer are closely matched. Furthermore, the anisotropy of thesecond free layer is large enough to assure a thermal factorK_(U)M_(S)V/k_(B)T>40, where K_(U) is the anisotropy constant, M_(S) isthe saturation magnetization, V is the second free layer volume, k_(B)is the Boltzmann constant, and T is temperature.

The write current passes from a first bit line through a first CPP cellwithin the first CPP/MTJ sub-cell and into the conductive spacer whichis connected through a transistor to a conductive spacer in a second CPPcell within a second CPP/MTJ sub-cell. In the second CPP cell, the writecurrent passes from the conductive spacer through the second CPP cell toa second bit line that is grounded. The transistor is controlled by awrite word line. When the magnetization in the second free layer in aCPP/MTJ sub-cell is switched by the write current, magnetic couplingwill force the first free layer to switch magnetization as well andthereby remain anti-parallel to the magnetization in the second freelayer. A high write current does not pass through the MTJ cell andavoids a tunnel barrier breakdown issue that has affected earlierSpin-RAM designs.

The bottom electrode in each CPP/MTJ sub-cell is connected to groundthrough a read transistor that is controlled by a read word line. Duringa read process, the first bit line and second bit line are biased at acertain voltage and when the read word line is turned on, a senseamplifier is used to sense the resistance difference between the firstMTJ cell and the second MTJ cell. If the first MTJ cell has a resistanceless than that of the second MTJ cell, then the bit cell is said to havea “1” memory state. When the first MTJ cell has a resistance greaterthan that of the second MTJ cell, the bit cell is said to have a “0”memory state.

The MTJ cell, conductive spacer, and CPP cell in each CPP/MTJ sub-cellmay be sequentially formed on the bottom electrode by a sputterdeposition process. Preferably, each MTJ cell is comprised of a seedlayer, AFM layer, pinned layer, tunnel barrier layer, free layer, and acapping layer that are sequentially formed on a bottom electrode. Thetunnel barrier layer may be an AlOx or MgOx layer, for example, that isformed by oxidizing an Al or Mg layer in an oxidation chamber within asputter deposition tool. Each CPP cell has a stack including a seedlayer, free layer, spacer, pinned layer, AFM layer, and capping layerformed sequentially from bottom to top on the conductive spacer.

The present invention anticipates that the CPP cell and MTJ cell may beindependently optimized so as to achieve a low write current in the CPPcell and a high dR/R in the MTJ. For instance, the CPP cell preferablyhas a larger size than the MTJ cell in order to take advantage of alarge demagnetization field from the CPP free layer and thereby moreefficiently switch the magnetic state in the MTJ free layer.Furthermore, the CPP cell may have a different shape than the MTJ cell.In one embodiment, the CPP cell has a large shape anisotropy derivedfrom an elliptical or eye shape while the MTJ cell has an essentiallyround shape to promote a low anisotropy. A MTJ stack of layers may belaid down and patterned to form a MTJ cell followed by formation of afirst dielectric layer adjacent to the MTJ cell. Subsequently, a seconddielectric layer may be formed on the first dielectric layer and thenthe conductive spacer and write word line may be formed in the seconddielectric layer. Thereafter, the CPP stack is deposited and patternedto form a CPP cell on the conductive spacer. Next, a third dielectriclayer is deposited and made coplanar with the CPP cell. The bit line isthen formed on the CPP cell by a conventional method.

The present invention also encompasses an embodiment wherein one or bothof the CPP cell and MTJ cell have a dual spin valve configuration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a graph depicting a switching field asteroid for aconventional MRAM while FIG. 1 b is a top view showing a conventionalmemory cell with a MTJ cell formed between a bit line and a word line.

FIG. 2 is cross-sectional view of a prior art Spin-RAM device.

FIG. 3 is a cross-sectional view of a prior art Spin-RAM structure inwhich a conductive spacer carries a write current between bit lines andproduces a spin transfer interaction between a magnetic polarizer and afree layer in a MTJ element.

FIG. 4 a depicts a CPP/MTJ sub-cell according to one embodiment of thepresent invention and shows the direction of write current through aconductive spacer between a CPP cell and a MTJ cell to an adjacentCPP/MTJ sub-cell within the same bit cell.

FIG. 4 b is a cross-sectional view of a MTJ cell in the CPP/MTJ sub-cellof FIG. 4 a and FIG. 4 c is a cross-sectional view of a CPP cell in theCPP/MTJ sub-cell of FIG. 4 a.

FIG. 5 is a cross-sectional view of two CPP/MTJ sub-cells in a bit cellaccording to the present invention and shows a write word line used tocontrol a transistor that connects adjacent conductive spacers.

FIG. 6 is an electrical diagram showing two adjacent bit cells accordingto one embodiment of the present invention, and a sense amplifier usedfor the read process.

DETAILED DESCRIPTION OF THE INVENTION

One aspect of the present invention is a CPP/MTJ sub-cell configurationwithin a Spin-RAM device that provides for a separation of write andread pathways so that the writing process and reading process can beindependently optimized. In the CPP and MTJ cells, the exemplaryembodiment depicts a single spin valve structure. However, the presentinvention also encompasses a CPP/MTJ sub-cell wherein one or both of theCPP cell and MTJ cell have a dual spin valve configuration. Devicesbased on this technology may be referred to as Spin-RAM or spin-transferMRAM devices. Another aspect of the present invention is a wiring schemecomprised of a plurality of bit cells, word lines, and bit lines thattakes advantage of a pair of CPP/MTJ sub-cells within each bit cellwhich enables the reading process to be more efficient. Those skilled inthe art will appreciate that the terms “magnetic moment” and“magnetization direction” may be used interchangeably.

First, the CPP/MTJ sub-cell structure will be described. Referring toFIG. 4 a, a cross sectional view of a Spin-RAM structure 50 is shownthat includes a bottom electrode 51 formed on a substrate (not shown), aMTJ cell 60 on the bottom conductor, a conductive spacer 70 on the topsurface of the MTJ cell, a CPP cell 80 on the conductive spacer, and abit line 90 above the conductive spacer. The spin transfer effect thatswitches the magnetic direction of a free layer within the CPP cell 80is generated by a write current 92 that flows from the bit line 90through the CPP cell 80 and into the conductive spacer 70. The writecurrent 92 is carried to an adjacent CPP/MTJ sub-cell (not shown) withinthe same bit cell through a transistor 130.

Referring to FIG. 4 b, the MTJ cell 60 is shown. In one embodiment, theMTJ cell 60 has a bottom spin valve configuration in which a seed layer61, AFM1 layer 62, AP1 layer 63, coupling layer 64, AP2 layer 65, tunnelbarrier layer 66, free layer (FM1) 67, and capping layer 68 aresequentially formed on the bottom electrode (not shown). The bottomelectrode 51 and MTJ stack of layers 61-68 may be formed in an AnelvaC-7100 thin film sputtering system or the like which typically includesthree physical vapor deposition (PVD) chambers each having five targets,an oxidation chamber, and a sputter etching chamber. At least one of thePVD chambers is capable of co-sputtering. Usually, the sputterdeposition process involves an argon sputter gas and the targets aremade of metal or alloys to be deposited on a substrate. The bottomelectrode 51 and overlying MTJ layers may be formed after a single pumpdown of the sputter system to enhance throughput.

The seed layer 61 may be made of NiCr or other suitable materials suchas NiFe, or NiFeCr that promotes a smooth and densely packed growth insubsequently formed MTJ layers.

The AFM layer 62 may be comprised of MnPt or other suitable materialssuch as IrMn, NiMn, OsMn, RuMn, RhMn, PdMn, RuRhMn, or MnPtPd that areeffective in pinning the adjacent AP1 layer 63 in a certain directionwhich is the x-axis in the exemplary embodiment. Thus, when the AFMlayer 62 is magnetically aligned in the x-axis direction, the AP1 layer63 will also be pinned in the x-axis direction.

The AP1 layer 63, coupling layer 64, and AP2 layer 65 form a syntheticanti-ferromagnetic (SyAF) pinned layer. Use of a SyAF pinned layer inthe MTJ cell not only improves thermal stability but also reduces theinterlayer coupling field (offset field) applied to the free layer (FM1)67. Both the AP1 layer 63 and AP2 layer 65 may be comprised of CoFe, forexample. The magnetic moment of the AP2 layer 65 is pinned in adirection anti-parallel to the magnetic moment of the AP1 layer 63. Aslight difference in thickness between the AP2 and AP1 layers produces asmall net magnetic moment for the SyAF pinned layer along the x-axis.Exchange coupling between the AP1 layer 63 and the AP2 layer 65 alsoknown as the reference layer is facilitated by a coupling layer 64 thatis preferably comprised of Ru.

Above the AP2 layer 65 is formed a thin tunnel barrier layer 66 that maybe made of AlOx, AlTiOx, or MgOx, for example. Generally, MgOx ispreferred because the combination of a CoFeB AP2 layer 65 and a MgOxtunnel barrier 66 yields a higher dR/R than achieved with other tunnelbarrier layers. Unlike a method commonly used in the prior art where aMgOx tunnel barrier is formed by a sputter deposition method, theinventors advantageously employ a procedure where a Mg layer about 6 to8 Angstroms thick is deposited followed by an in-situ radical oxidation(ROX) or natural oxidation (NOX), and then deposition of an additionalMg layer about 2 to 6 Angstroms thick. The tunnel barrier layer 66 hasexcellent smoothness and uniformity in part because of the smoothunderlying MTJ layers. The ROX or NOX process is preferably performed inan oxidation chamber within the sputter deposition system. In oneembodiment, the ROX process is comprised of a RF power of about 300 to500 Watts and an oxygen flow rate of 0.4 to 0.8 standard liters perminute (slm) and preferably about 0.6 slm for a period of about 15 to 50seconds. Optionally, the NOX process may be comprised of a 1 torrpressure and an oxygen flow rate of from 0.1 to 1 slm, and preferablyabout 1 sim for about 60 to 120 seconds to oxidize the Mg layer on theAP2 layer 65. The process conditions for the ROX and NOX processes areselected to achieve a certain RA value specified in the design of a MTJcell. Typically, if a low RA of less than about 10 ohm-um² is desired,then a NOX process is selected. One or both of the oxygen flow rate andprocess time may be decreased within the ranges mentioned above todecrease the RA value.

The free layer (FM1) 67 may be a single layer or a composite comprisedof soft magnetic materials such as an alloy or ternary of Ni, Fe, Co,and B. The free layer 67 has a magnetic moment that may be aligned alongthe −x axis or +x-axis direction depending on the state to which it iswritten to. A lower resistance state is observed when the magneticmoment of the AP2 layer 65 and free layer 67 are aligned in the samedirection. An important feature of the present invention is that thefree layer 67 preferably has a low anisotropy of <10 Oe, and morepreferably <5 Oe so that the first free layer (FM1) does not exert astrong demagnetization field on the second free layer (FM2 in FIG. 4 c)and thereby stabilize the FM2 free layer 82 which would makespin-transfer switching by a write current more difficult. Furthermore,a low anisotropy enables the FM1 free layer 67 to be more easily rotatedunder the influence of the FM2 free layer 82 when the latter is switchedduring a spin-transfer write process. Since the shape of a MTJ cell istypically is one of the key factors in determining anisotropy in the FM1free layer 67 (besides the material selection), a substantially circularcell shape is preferred in order to achieve an anisotropy of <5 Oe. Itshould be understood that an elliptical shape, eye shape, or otherrounded shape cell are likely to lead to a higher anisotropy thandesired. However, non-circular shapes are not precluded from being usedin this embodiment because certain conditions wherein a first axis isslightly elongated compared with a second axis in the non-circular shapemay still afford a low anisotropy of <5 Oe, or at least <10 Oe.

Preferably, the capping layer 68 is made of one or more of Ta, Ru, orother materials that enable the MTJ cell to achieve a high dR/R. A hightemperature thermal annealing in the range of 250° C. to 350° C. istypically employed in the presence of a large external magnetic field ofabout 5000 to 10000 Oe to set the magnetization direction of the AFM1layer 62. As a result, the magnetic moments of the AP1 layer 63 and AP2layer 65 will also be set by influence from the AFM1 layer 62 and AP1layer 63, respectively.

Returning to FIG. 4 a, the conductive spacer 70 between the MTJ cell 60and CPP cell 80 is made of a highly conductive material such as Cu, Au,Ag, or Al, for example. In an embodiment wherein the MTJ cell 60 and CPPcell 80 have different shapes, the conductive spacer 70 may have thesame shape as either the MTJ cell or CPP cell. The conductive spacer 70may be deposited by a physical vapor deposition (PVD) process in asputter deposition chamber. A chemical mechanical polish (CMP) processmay be employed to smooth the top surface of the conductive spacer 70before the CPP stack of layers are laid down.

Referring to FIG. 4 c, the CPP cell 80 is illustrated. In oneembodiment, the CPP cell is comprised of a seed layer 81, FM2 free layer82, CPP spacer 83, AP3 layer 84, coupling layer 85, AP4 layer 86, AFM2layer 87, and a capping layer 88 formed in consecutive order from bottomto top on the conductive spacer 80. The capping layer 88 may be regardedas a top electrode. Preferably, the CPP cell 80 has a larger size from atop view (not shown) than the MTJ cell 60. A larger CPP size enables theFM2 free layer 82 to exert a larger demagnetization field on the FM1free layer 67 (FIG. 4 b) and make rotation of the FM1 free layermagnetization direction (switching) easier to accomplish during a writeprocess. In addition, the shape of the CPP cell 80 from a top-down viewis preferably elliptical, eye-shape, or another type of rounded shapethat drives a high shape anisotropy in the FM2 free layer 82 which inturn causes a high demagnetization field on the FM1 free layer 67.

Preferably, the seed layer 81 is comprised of Cu, Au, Ag, Al, or othermaterials that are known in the art as non-spin sinkers. In other words,a non-spin sinker will improve (i.e. reduce) or at least not degrade thedamping ratio of the FM2 free layer 82. The seed layer 81 also providesfor better magnetic properties in overlying magnetic layers and a lowdamping ratio for the FM2 free layer 82.

Another key feature of the present invention is the FM2 free layer 82which may be a single layer or a composite material. Preferably, the FM2free layer 82 has a low damping ratio to promote the spin-transfereffect in the CPP cell 80 during a write process. Furthermore, the FM2free layer 82 should have a substantial anisotropy that is sufficientlylarger than that of the FM1 free layer 67 in order to exert a largedemagnetization field on the FM1 free layer that will facilitatemagnetization switching in the FM1 free layer when the magnetic momentof the FM2 free layer is reversed. In one embodiment, the FM2 free layer82 preferably has an anisotropy>50 Oe, and more preferably >100 Oe whichis achieved in part by an elliptical shape, eye shape, or anotherrounded shape as mentioned previously. Preferably, the FM2 free layer 82anisotropy is from 3 to 1000 times greater than that of the FM1 freelayer 67. Additionally, the FM2 free layer 82 anisotropy should besufficiently large to assure a thermal factor K_(U)M_(S)V/k_(B)T>40where Ku is the anisotropy constant, Ms is the saturation magnetization,V is the volume of the FM2 free layer, k_(B) is the Boltzmann constant,and T is the temperature. A thermal factor greater than 40 is necessaryto assure a minimum length for data retention of 10 years as statedpreviously.

The CPP spacer 83 is a non-magnetic material and is preferably a thinmetallic layer such as Cu, Au, or other highly conductive materialshaving a thickness of about 20 to 100 Angstroms, and more preferablybetween 25 and 60 Angstroms, in order to provide better spin transferefficiency which is equivalent to easier spin-transfer induced switchingof the FM2 free layer 82 magnetization. If the CPP spacer 83 is lessthan about 20 Angstroms thick, then the spacer may not be sufficientlythick to prevent interdiffusion between the FM2 free layer 82 and AP3layer 84. When the CPP spacer 83 is thicker than about 100 Angstroms,then an undesirable reduction in the degree of spin polarization occursdue to scattering.

Similar to the MTJ cell 60, the AP3 layer 84, coupling layer 85, and AP4layer 86 form a SyAF pinned layer in which the AP3 layer serves as areference layer for the CPP cell 80 and the AP4 pinned layer has amagnetic moment fixed in a certain direction by coupling with theadjacent AFM2 layer 87. In the exemplary embodiment, the magnetizationdirection of the AP3 layer 84 is along the −x axis, and the AP4 layer 86and AFM2 layer 87 are magnetically aligned in the +x axis direction. TheAP3 layer 84 and AP4 layer 86 may be a single layer or a composite ofmagnetic materials such as an alloy or ternary comprised of two or moreof Ni, Fe, Co, and B. The magnetic moments of the AP3 layer 84 and AP4layer 86 are nearly the same magnitude but aligned in oppositedirections for the same reasons as described earlier with respect to AP1layer 63 and AP2 layer 65. The coupling layer 85 may be made of Rh, Ru,or Ir. A high temperature thermal annealing in the range of 250° C. to350° C. is typically employed in the presence of a large externalmagnetic field of about 5000 to 10000 Oe to set the magnetizationdirection of the AFM2 layer 87. Note that the annealing of the AFM2layer 87 may be performed simultaneously with the annealing of AFM1layer 62 since both are magnetically aligned in the +x-axis direction.The AFM2 layer 87 and capping layer 88 may be comprised of the samematerials as in AFM1 layer 62 and capping layer 68, respectively.

In each bit sub-cell, the CPP cell is formed above the MTJ cell and hasa larger area size from a top view (not shown) than the MTJ cell. TheCPP cell from a top view preferably has an area 10% to 100% greater thanthat of the MTJ, and more preferably 30 % to 60% greater than the MTJcell area. Although the area size of the CPP cell relative to the MTJcell is important, it should be understood that the shape of the area isalso a critical factor in determining the magnitude of free layeranisotropy as described earlier. Both the shape and area size of the FM1free layer 67 and FM2 free layer 82 are optimized to provide a FM2 freelayer anisotropy that is preferably about 3 to 10 times greater than theFM1 free layer anisotropy, and more preferably about 3 to 1000 timesgreater than the FM1 free layer anisotropy. Since the FM2 free layer 82(FIG. 4 c) has a relatively large uniaxial anisotropy, its magnetizationdirection remains along the anisotropy axis, either parallel oranti-parallel to the AP₃ layer 84 (reference layer) magnetizationdirection. Because the FM1 free layer 67 (FIG. 4 b) has a relativelysmall anisotropy, the FM1 free layer experiences a demagnetization fieldfrom the FM2 free layer magnetization. Note that the magnetization ofthe FM1 free layer 67 will be anti-parallel to the magnetization of theFM2 free layer 87 and thus form a partial flux closure which is amagnetic coupling interaction.

To write data into a bit sub-cell 50 as shown in FIG. 4 a, the electriccurrent 92 is either injected from the bit line 90 and then flows acrossonly the CPP cell 80 into conductive spacer 70 and then out through awrite word line 110 to an adjacent bit sub-cell and out to another bitline (not shown), or in a reverse direction to current 92. When the freelayer in CPP cell 80 is set along one of its anisotropy directions byspin-transfer induced switching, the demagnetization field from the CPPfree layer will rotate the magnetization direction in the MTJ free layerto remain anti-parallel to the CPP free layer magnetization direction.

To read from a bit sub-cell 50, the read current flows from the bit line90 to the bottom electrode 51. Although the CPP cell 80 and MTJ cell 60are series connected, the resistance of the MTJ cell, which is severalorders of magnitude larger than that of the CPP cell, is directly readand compared with a reference cell resistance.

One benefit of the CPP/MTJ sub-cell design of the present invention isthat the write path and read path are separated so that a high writecurrent does not flow across the MTJ cell and a key reliability problemin Spin-RAM related to tunnel barrier breakdown is no longer an issue. Asecond advantage is that the MTJ cell 60 can be optimized by selectingmaterials for the MTJ cell layers 61-68 to achieve a higher dR/R andlower resistance variance without concern for adversely affecting thetunnel barrier breakdown voltage, or the magnitude of the MTJ free layerdamping ratio. Likewise, the CPP cell 80 can be independently optimizedby selecting materials for the CPP cell layers 81-88 to achieve a lowerdamping ratio of the FM2 free layer 82 and a smaller switching current.

In one embodiment, the MTJ cell layers 61-68 may be sequentially formedin a sputter deposition tool and are then patterned by a conventionalprocess. Next, a first dielectric layer (not shown) is deposited andpolished by a chemical mechanical polish (CMP) process to be coplanarwith the capping layer 68 in the MTJ cell 60. Subsequently, a seconddielectric layer (not shown) may be formed on the first dielectric layerand then the conductive spacer 70 and write word line 110 wiring may beformed in the second dielectric layer by a well known process. A CMPprocess may be used to planarized the conductive spacer 70 and writeword line 110. Thereafter, the CPP stack of layers 81-88 is formed onthe conductive spacer 70 and is patterned by a conventional process toform the CPP cell 80. A third dielectric layer (not shown) is depositedand made coplanar with the CPP cell. The bit line 90 is then formed onthe CPP cell 80 and third dielectric layer by a conventional method.

Another aspect of the present invention is to provide a Spin-RAM circuitthat employs the CPP assisted writing scheme as described in a previousembodiment, and also includes an improved read architecture. Referringto FIG. 5, an improved read process is achieved by having two bitsub-cells 50 a, 50 b in each bit cell wherein a first bit sub-cell 50 ais written to a “0” and a second bit sub-cell 50 b is written to a “1”to yield a (0,1) resistance state for the bit cell 120. Alternatively,the first bit sub-cell 50 a may be written to a “1” and the second bitsub-cell 50 b may be written to a “0” to give a (1,0) resistance state.In the reading process, the resistance values are compared within eachbit cell to determine the final bit state. In one embodiment, when thefirst bit sub-cell 50 a has a greater resistance than the second bitsub-cell 50 b, the bit cell 120 is said to have a “0” resistance stateand when the first bit sub-cell 50 a has a lower resistance than thesecond bit sub-cell 50 b, the bit cell 120 has a “1” resistance state.This method of reading bit cell 120 has an advantage in read reliabilityand speed compared with prior art structures where a resistance in acertain bit cell is compared with the resistance in a reference cellthat may be a substantial distance from the bit cell to be read.

The pathway of the write current 92 is illustrated in more detail inFIG. 5. In one embodiment, the write current 92 originates in bit line90 a and passes through the CPP cell 80 and conductive spacer 70 a inthe first bit sub-cell 50 a before flowing out through transistor 130and into a conductive spacer 70 b in the second bit sub-cell 50 b.Simultaneously, a current is injected to write word line (not shown)which controls transistor 130. The write current 92 and the currentapplied to write word line occur at the same time to force a currentfrom conductive spacer 70 a to conductive spacer 70 b. The write current92 flows from conductive spacer 70 b up through CPP cell 80 b and intobit line 90 b. Alternatively, the write current 92 may flow in theopposite direction of write current 92 and move from bit line 90 b tobit line 90 a when a voltage is applied to transistor 130 that forces acurrent to flow from conductive spacer 70 b to conductive spacer 70 a asunderstood by those skilled in the art.

Referring to FIG. 6, an electrical diagram is provided that depicts twobit cells 200 a, 200 c which represent the Mth bit cell and Nth bitcell, respectively, in a Spin-RAM array comprised of a plurality of bitcells. The outer boundary of each bit cell is shown by the dashed lines.In bit cell 200 a, there is a first sub-cell comprised of a first CPPcell 80 a and a first MTJ cell 60 a separated by a conductive spacer 70a, and there is a second sub-cell comprised of a second CPP cell 80 band a second MTJ cell 60 b separated by a second conductive spacer 70 b.A first write word line 110 a controls a transistor 130 a that connectsthe conductive spacers 70 a, 70 b. The bottom electrodes 50 a, 50 b infirst and second sub-cells are connected through read transistors 140 a,150 a, respectively, to ground. Orthogonal to the write word lines 110a, 110 c direction are two bit lines 90 a, 90 b which are connected tothe top electrodes (capping layers) 88 a, 88 b, respectively, in thefirst sub-cell and second sub-cell.

Bit cell 200 c has a first sub-cell comprised of a third CPP cell 80 cand a third MTJ cell 60 c separated by a conductive spacer 70 c, andthere is a second sub-cell in bit cell 200 c comprised of a fourth CPPcell 80 d and a fourth MTJ cell 60 d separated by a conductive spacer 70d. A second write word line 110 c connects conductive spacers 70 c, 70 dand is controlled by a transistor 130 c. The bottom electrodes 50 c, 50d in the two sub-cells are connected through transistors 140 c, 150 c,respectively, to ground. Bit lines 90 a, 90 b are connected to the topelectrodes (capping layers) 88 c, 88 d, respectively, in the firstsub-cell and second sub-cell within bit cell 200 c.

To write “1” to bit cell 200 c, bit line 90 a functions as a writecurrent source while bit line 90 b is grounded (not shown). When thewrite word line 110 c is turned on, a write current (not shown) flowsfrom bit line 90 a through CPP cell 80 c (from top to bottom) and thenthrough CPP cell 80 d (from bottom to top) and finally to bit line 90 b.The write current also passes through conductive spacer 70 c, write wordline 110 c, and conductive spacer 80 c when flowing between CPP cell 80c and CPP cell 80 d. Since the write current flows across CPP cells 80c, 80 d in opposite directions, the CPP cells 80 c, 80 d are written toopposite magnetization states. Due to the demagnetization field exertedby CPP cells 80 c, 80 d, the MTJ cells 60 c, 60 d are rotated from aprevious state and also have opposing magnetization directions. Forexample, when MTJ cell 60 c has a higher resistance, MTJ cell 60 d has alower resistance. Therefore, writing a “1” to bit cell 200 c results inresistance states for CPP cells 80 c, 80 d of “0” and “1”, respectively,and resistance states for MTJ cells 60 c, 60 d of “1” and “0”,respectively.

To write “0” to bit cell 200 c, bit line 90 b functions as a writecurrent source while bit line 90 a is grounded. When the write word line10 c is turned on, a write current flows from bit line 90 b through CPPcell 90 d (from top to bottom) and then through CPP cell 90 c (frombottom to top), and finally to bit line 90 c. Thus, the resistancestates of CPP cells 80 c, 80 d are “1” and “0”, respectively, and theresistance states of MTJ cells 60 c, 60 d, are “0” and “1”,respectively, when the process of writing a “0” to bit cell 200 c iscompleted.

To read data in the bit cell 200 c, both bit line 90 a and bit line 90 bare biased at a certain voltage of about 0.05 to 0.5 volts. When theread word line 105 c is turned on, the sense amplifier 160 is used tosense the difference in resistance between MTJ cell 60 c and MTJ cell 60d. When the MTJ cell 60 c has a greater resistance than the MTJ cell 60d, the bit cell 200 c is said to have a “0” resistance state. On theother hand, when MTJ cell 60 c has a lower resistance than MTJ cell 60d, the bit cell 200 c is said to have a “1” resistance state. Therefore,the read process according to the present invention is greatly improvedover the prior art by enabling a shorter read time and greater readreliability than conventional read schemes which rely on a referencecell outside the bit cell. In the prior art, the resistance (R1) of aMTJ cell is compared with the resistance (Rref) of a reference cell. Areference cell is specifically designed so that its resistance, Rref, isthe averaged resistance value between the higher resistive state and thelower resistive state. A “1” resistance state is determined when(R1−Rref) is <0. Choosing the averaged resistance value between thehigher resistive state and the lower resistive state as Rref can resultin an absolute value for (R1−Rref) that is quite small and can easilycause a “1” state to be falsely read as a “0” state, or vice versa. Readerror is substantially minimized in the embodiment described hereinbecause the “0” and “1” states are determined by a simpleR_(MTJ1)>R_(MTJ2) or R_(MTJ2)>R_(MTJ1) rather than measuring a certainmagnitude of R1 and R2.

While this invention has been particularly shown and described withreference to, the preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of this invention.

1. A spin-transfer MRAM structure comprising a bit cell array whereineach bit cell has two sub-cells which are a first sub-cell and a secondsub-cell, and each sub-cell comprises: (a) a MTJ cell formed on a bottomelectrode wherein the MTJ cell has a first free layer and a firstsynthetic pinned layer that are separated by a tunnel barrier layer,said MTJ cell has a near zero anisotropy, and said first free layer hasa magnetization that is oriented anti-parallel to a magnetizationdirection in an overlying second free layer in a CPP cell such that whenthe second free layer is switched by a write current, the first freelayer is also switched through a magnetic coupling interaction with thesecond free layer; (b) a conductive spacer layer formed on the MTJ cell;and (c) a CPP cell with a top electrode and formed on the conductivespacer wherein the CPP cell is comprised of a second free layer and asecond synthetic pinned layer separated by a non-magnetic spacer, andsaid CPP cell has a substantial anisotropy, and said second free layerhas a magnetization that is switched either by a write current flowingfrom a bit line contacting the top electrode through the CPP cell andthen to the conductive spacer, or from the conductive spacer through theCPP cell and then to the bit line contacting the top electrode; andwherein the MTJ cell and CPP cell in each sub-cell have a differentresistance state, the two MTJ cells in each bit cell have differentresistance states, and the two CPP cells in each bit cell have differentresistance states.
 2. The spin-transfer MRAM structure of claim 1wherein the conductive spacer in the first sub-cell is connected to theconductive spacer in the second sub-cell through a write word linecontrolled by a write transistor.
 3. The spin-transfer MRAM structure ofclaim 1 wherein the bottom electrode in each sub-cell is connected toground through a read transistor.
 4. The spin-transfer MRAM structure ofclaim 1 wherein the MTJ cell is a single spin valve and the CPP cell isa dual spin valve in a sub-cell.
 5. The spin-transfer MRAM structure ofclaim 1 wherein the MTJ cell is a dual spin valve and the CPP cell is asingle spin valve in a sub-cell.
 6. The spin-transfer MRAM structure ofclaim 1 wherein the MTJ cell is a dual spin valve and the CPP cell is adual spin valve in a sub-cell.